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# How to determine the units of a rate constant? Help please? watch

1. If the rate constant for a reaction is the gradient of a graph which is the cube root of mass (g) versus time (mins) what are the units for the rate constant? (The cube root of mass is on the y axis and time is on the x axis). I don't know what the order of the reaction is but its just the weight of a sweet dissolving in water over time if that's any help? :s thanks in advance!
2. Without hearing the shape of the graph there is absolutely no chance we can tell you what the units of the rate constant are.

More precisely, we need the order of the reaction. From there, it's pretty easy; work out the overall order of the reaction by summing the orders with respect to each component in your rate equation, and if we call this overall order 'n', then k will have the units of X1-nt-1 (where X can be any unit of amount, moldm-3, g, Pa, atm, torr, whatever you want; and t can be any unit of time, s, min, hour, whatever you want). Both units would have to be consistent with those by which you measured the rate.

What you have to do is find the gradient at different points on your graph and this is the rate of the reaction at different times. If we have the rate of reaction at two different values of concentration, we can easily work out the order of the reaction. And then you can find the units of k.

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Updated: April 14, 2013
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